1. Field of the Invention
The present invention relates to a method and equipment for assembling a plurality of components using a robot. Particularly, the invention is concerned with a components assembling method and equipment wherein components are each pushed against two or more reference planes to correct the posture thereof.
2. Description of the Prior Art
The process of joining faces of components constituted by many faces to assemble the components into a product is often carried out by manual labor, and attempts have heretofore been made to automate the said process by the use of a robot. Also as to operations to be performed by the robot, there are required contents corresponding to six axes of freedom or more, and to meet this requirement there is now a tendency to use a robot having joints of six axes or so. Therefore, for assembling a plurality of components into a desired product with a predetermined certain accuracy, using a robot, it is necessary that the axes of the robot for determining the position and posture of the robot be moved with a predetermined certain accuracy.
According to a conventional method for assembling such components using a robot, the position to which the robot is to move is given to each axis of the robot by teaching, and the position thus taught is reproduced as a motion of the robot.
On the other hand, as components to be assembled using a robot there are not only machined components but also components for canning. It goes without saying that a high accuracy is required for assembling machined components. But in assembling components for canning, the cutting and bending accuracy for plates is not so strict as the machining accuracy, so in order to obtain a weld assembly of a good quality, it is important to minimize the contact clearance between plates in a groove for welding.
However, a relative positional relation between a robot grasping a component with its hand and the component varies in a small range, depending on, for example, the component machining accuracy or the component grasping accuracy. Thus, an uncertain factor is involved.
Such component machining error and robot component grasping error vary each individually and are extremely small amounts in comparison with motions of the robot, so it is impossible to reflect all of these points in the foregoing teaching of robot motions. For this reason, in the case of assembling plural components by contacting the components at respective faces, there arises the necessity of assembling the components while making such contact faces compatible with each other to correct a displacement caused by a component machining error or a component grasping error.
According to a known method for correcting such a displacement, which method intends to assemble components with a high accuracy, there is used a component positioning device including a hydraulic cylinder or the like. In this known method, each component is loaded to the positioning device, and after correction of a displacement, is again grasped by a robot at a predetermined position.
However, in assembling components with use of such a positioning device, it is required to use such a positioning device for each of similar components. Therefore, the case where the aforesaid known method can actually be applied is limited to the case where the number of components is small and the components are of limited sizes and shapes or to the case where products assembled by using components are mass-produced. Even if the above known method is applied to any of these cases, there arises a problem of increased equipment cost or a problem related to tact time.
Where a robot is to be used in fitting operation, it is usually required to use a positioning device at the time of supply of components, and even if components are supplied under positioning, the strictness of fit tolerance is to a further extent than the robot positioning accuracy. Additionally, for avoiding interference with a reaction force created in fitting operation, there is used a mechanical float or a compliance control.
The mechanical float permits face contact of different components, but the mechanism for holding the position and posture of abutted components is complicated and a mechanical float capable of controlling the aforesaid fitting operation in a six-axis structure has not been commercialized yet. In the use of a compliance control each component has six axes of freedom in a space, but five axes of freedom are restrained from a shaft-hole relation in fitting, and it is only one axis of freedom that can adjust motion in fitting operation. Thus, it is easy to effect control. However, controlling method and apparatus suitable for fitting, which employ such compliance control, are not applicable to the abutting operation for minimizing the contact clearance of faces generated in the mechanical assembly or canning assembly proposed herein, because the degree of freedom is not limited up to completion of the abutment.
In addition to the above mechanical float and compliance control there also has been commercialized a servo float function as a function which can adjust robot motions in a flexible manner and which is applicable to the aforesaid operation of abutting components between their faces. This servo float function is a robot controlling method in which, as described in Japanese Patent Laid Open No. 210251/95, a control system for a servo motor as a robot drive source is formulated using a feedback control system, and the rigidity of robot motions is varied by changing and adjusting various gains in the feedback loop for the motor, thereby making it possible to keep constant the flexibility of robot motions.
At present, as cases where the above servo float function is applied to assembling components under the control of robot motions, there are mentioned 1) a case where an external force exerted on the robot is turned aside at the sacrifice of positioning accuracy, 2) a case where, in assembling components, the posture of the robot itself is changed with a force which the robot generates, to follow the components, and 3) a case where the robot exerts a certain force on each component (Yasukawa Denki Technical Report, Vol. 59, No. 2). It is the case 2) that is close to the present invention. However, that the posture of the robot itself changes means that a target position which a component is to assume changes, even if it is possible to follow the component. Thus, it is impossible to assemble components with a high accuracy.
As described above, in the prior art where a component positioning device is used in addition to a robot and an assembly stand, there is no flexibility for the change of components, and the tact time becomes long. In the mechanical float, compliance control and servo float control not using a component positioning device, a component supply error, a component size error and a robot grasping error can be absorbed by a positioning action between components as in fitting operation, but in other cases there is the problem that it is impossible to ensure a high positioning accuracy even though it is possible to let the robot follow the contact faces of components.
Therefore, when a plurality of components each of which is not always satisfactory in machining accuracy are to be mutually contacted at their faces using a robot of six or more axes and are to be assembled at a target position within an allowable accuracy range while minimizing the clearance between the contact faces, it is desired that the components be assembled while adjusting the motion of each robot axis in a flexible manner to make the contact faces of components compatible with each other and that a plurality of components be assembled successively while maintaining a predetermined assembling accuracy. Besides, while maintaining such predetermined assembling accuracy, it is necessary that the cost of required equipment should not increase nor should the working time become long.